Bumps @version to 5.0.0, promotes CHANGELOG.md's ## [Unreleased]
section to ## [5.0.0] - 2026-08-12 behind a fresh empty Unreleased
header, and moves the README install pin to ~> 5.0.

The major bump is carried by the function-provider rewrite: a custom
function's second argument is now the %Predicator.Context{} struct
rather than the bare data map, and if, else, while and undefined
became reserved words. The release also ships ISA v4's cast, v5's
jump and pop_jump_if_falsy, and v6's jump_backward with the loop
budget.

Trims the trailing clause of the ADR-0014 changelog entry, which
said the entry was waiting under ## [Unreleased] and stops
describing anything once it sits under a version header.

Refs: px-6s0